The Cold Trap

I’ve read it’s important to have a cold trap and a baffle between the diffusion pump and the chamber to prevent back streaming of pump oil onto the optics. Normally the cold trap uses liquid nitrogen as the refrigerant. I’ve always wanted to play with liquid nitrogen, but it didn’t seem practical. So I did some research and found the following site by Dr. Scott Teare http://www.ee.nmt.edu/~teare/mwo_al_images.htm . After a few emails with Dr. Teare I realized I could build something like this J The Mount Wilson Observatory uses refrigerated glycol as the coolant, so I got to thinking about using this old refrigerator pump I had, to build my own cold trap, using R-22 as the coolant.

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Here’s what I built. Instead of using the conventional air-cooled freon condenser, I choose to go with a water cooled condenser. I figured I already needed running water to cool the diffusion pump, so why not use it to cool the freon system as well. The large copper pipe (attached to the garden hose) has inside it a length of ¼” pipe housing the high pressure freon, cooling it from a gas into a liquid. From there the cooled liquid freon goes thru a adjustable valve that replaces a conventional capillary tub or orifice. The valve gives me the ability to adjust the low side pressure, which directly controls the temperature of the cold trap. Works pretty well!. During testing I was able to get near –45c!

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In the above picture is a detail of the freon lines going into the adapter between the base plate and diffusion pump valve. I used a compression fitting with pipe threads on one end, drilled and tapped the adapter flange for the pipe threads, then drilled the compression fitting to allow the ¼” freon lines to travel thru and into the chamber. The compression nut now works to keep the air from entering the chamber. Was kind of tricky to install, but well worth the time!

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This image shows the copper coil inside the chamber area. I had originally planned on installing a chevron baffle on top of this coil, but have found no evidence of oil back streaming into the chamber. Consequently the baffle idea is on hold until such a time as it appears needed!